Diagnosing Yellow Leaves on Oak: Iron Chlorosis vs. Fungus in Houston

A Houston-area oak shows interveinal yellowing (green veins, yellow tissue)—classic iron chlorosis. A tree service diagnosed fungus and recommended deep-root chemicals. Here's how to distinguish and treat.

Iron Chlorosis Symptoms

• Yellowing between veins on youngest leaves first.
• Veins remain green.
• Progresses to browning, leaf drop, twig dieback in severe cases.
• Often accompanied by fungal leaf spots (secondary, not primary cause).

Primary Causes in Alkaline Soils

Houston's soils are often alkaline (pH > 7.0), locking up iron. Other factors: waterlogged soil, root rot, compaction, or true iron deficiency.

Effective Treatments

• Soil application: Iron sulfate at 1 lb per ½ inch trunk diameter, broadcast under canopy. Best in early spring.
• Deep-root injection: Chelated iron (EDDHA formulation—e.g., Sequestar 6%, Sprint 138, Millers FerriPlus) in holes 15–18 inches deep, 2 ft apart, in concentric circles from 3 ft out to 3 ft beyond drip line. Lasts 2–3 years.
• Foliar spray: Quick green-up but temporary; use chelated iron per label.

Ruling Out Other Issues

Manganese deficiency shows similar yellowing but with broad green bands along major veins and affects older leaves first. Herbicide damage, nitrogen, zinc, or boron deficiencies can also mimic chlorosis.

Professional Verification

Contact the Texas A&M Forest Service (Houston County) for an on-site diagnosis by a certified forester. They provide unbiased, science-based recommendations.

Iron chlorosis is manageable with the right product and timing—don't settle for a vague "fungus" diagnosis without soil testing.